Newshub political editor Patrick Gower says the 2017 election is about to become a battle of the bribes.
The Treasury will on Wednesday reveal how much money the next Government can expect to have to spend.
Gower says Labour and National will compete to offer up new spending to voters.
"It's the pre-election fiscal update. It's the opening of the books, or it's the 'show me the money' moment. This is when the Treasury says 'this is how much cash we've got.' And we're talking here $1.5 to $2 billion extra," he told the AM Show on Wednesday.
"That's going to be used for one thing - bribes. There will be a bribe-off. This election is tight. Steven Joyce will come up with his bribe that he'll give to Bill English to sell, and on the other side Jacinda Ardern and Grant Robertson will come up with their bribe."
Treasury's half-year update, released last December, forecast steadily rising surpluses reaching $8.5 billion by 2020/21.
But by the time Mr Joyce presented his first budget in May this year that had slipped to $7.2b by 2020/21, partly because of the Government's family income package.
Gower says the parties will hope to drive turnout with their hand-outs.
"We know what Labour want to do, it's to do with free education, it's to do with allowances, it's to do with something to get out there to those students, create some sort of youth-quake," he says.
"We don't know what National want to bribe the electorate with. We don't know what Steven Joyce is thinking. But, you've got to say, it will be around potentially tax-cuts."
Gower sees two groups Mr Joyce could target for their votes - middle to higher-income earners and superannuates.